The invention relates to detector apparatus of the two-terminal type supplied with rectified AC power and including a sensor associated with a control circuit, which apparatus are series-connected with the load to be controlled, and more particularly it relates to apparatus the supply voltage of which can vary in a wide range, e.g. from about 20 V to about 240 V, and to supply means, means for controlling the output current, and means for protecting against overloads or shorts, which are incorporated in such apparatus.
As an example, the apparatus may be a proximity or presence detector having a photocell, of the capacitive or other type.
In such apparatus, the load control is frequently carried out by gating at least one thyristor whenever the diode provides a control signal, which causes a current, so called output current, having an intensity sufficient to control the load, to flow through the load in the circuit.
These apparatus generally include, in addition to the sensor and the control circuit, means for rectifying the A.C current feeding the apparatus, resistors which are series-connected with the sensor-control circuit assembly and through with the latter is fed whenever the load is in its off-state, and at least one Zener diode connected across said assembly and in series with the thyristor so as to define a reference voltage.
In such a type of apparatus, it is important not to disturb the load operation by reducing the voltage across the apparatus whenever the load is being controlled. The voltage across the apparatus, then so-called "waste voltage", must be as low as possible. Furthermore, since an apparatus of this kind must operate under quite variable supply voltages, e.g. from about 20 V to about 240 V, it is necessary to control the current flowing through the circuit whenever the load is off so that its value remains negligible and substantially constant whichever is the above-defined supply voltage; this current is then referred to as "residual current".